Conventionally, various extracting methods for extracting a specific component from a material fluid have been known. One of the extracting methods is exemplified in Patent Literature 1.
In an extracting method disclosed in Patent Literature 1, mixer-settler units provided for a plurality of stages are used to extract neodymium and praseodymium as specific components from a nitric acid-based mixed solution containing the neodymium and the praseodymium as a material fluid to a quaternary amine solution as an extraction agent.
The mixer-settler unit for each stage includes a mixer part, a settler part, and a transfer part. In the mixer part, the introduced nitric acid-based mixed solution and quaternary amine solution are mixed with each other. The mixer part includes an agitator for agitating the introduced nitric acid-based mixed solution and quaternary amine solution to form an emulsion. The emulsion formed in the mixer part is introduced into the settler part. In the settler part, the introduced emulsion is settled to be separated into an organic phase and an aqueous phase. The transfer part transfers the organic phase having been separated in the settler part to the mixer part of the mixer-settler unit for a succeeding stage, and transfers the aqueous phase having been separated in the settler part to the mixer part of the mixer-settler unit for a preceding stage. The extraction of the neodymium and praseodymium from the nitric acid-based mixed solution to the quaternary amine solution is performed in an agitating step in the settler part and a settling step in the mixer part of the mixer-settler unit for each stage.
Meanwhile, there is a case in which the pH of a material fluid changes in accordance with progress in extraction of a specific component from a material fluid, and consequently an extraction efficiency of the specific component will change due to the change of the pH, depending on a type of extraction reaction. For example, the pH of the material fluid lowers in accordance with the extraction progress, and consequently the extraction efficiency of the specific component of the material fluid will deteriorate due to the lowering in the pH.
In the extracting method using the aforementioned mixer-settler units, some micro particles of the organic phase are dispersed into the aqueous phase due to the agitation in the mixer part. Hence, a considerably long time period is required to settle and separate the aqueous phase containing the dispersed micro particles of the organic phase into the organic phase and the aqueous phase in the settler part. Accordingly, the extraction progress continues until the separation is completed. The pH of the material fluid is high and the extraction progresses at a high extraction speed immediately after the material fluid and the extraction agent are mixed with each other. However, after an elapse of a predetermined time period, the pH of the material fluid lowers in accordance with the extraction progress so that the extraction progress slows down. During a large part of the long time period required to complete the separation in the settler part, the extraction progress is very slow or the extraction reaches and maintains an extraction equilibrium due to the pH of the material fluid that has already lowered. Accordingly, the time efficiency of the extraction process extremely deteriorates in this case.